Not kryptonite in the Fortress of Solitude—the noble gas trapped in air bubbles.

There’s a reason that one ice core drilled from the Greenland Ice Sheet has been referred to as The Two-Mile Time Machine—the annual layers of snow (compressed to ice) provide remarkable records of Earth’s climate and atmosphere. Studying past changes in atmospheric gases is just so much easier when you can pluck a bubble of air dutifully archived by the ice, which acts like a relative with an inconveniently large collection of National Geographics. The only problem with the “Two-Mile Time Machine” is that it’s only two miles long. Ice cores in Greenland can only go back about 130,000 years, and the oldest (so far) core from Antarctica goes back about 800,000 years.

If we could go a little further back in time, about 1.2 million years ago, we'd be able to examine a terribly interesting climatic transition. Prior to that time, glacial cycles were roughly 40,000 years long; after, we've experienced 100,000 year ones. We can study that period with some ocean sediment cores, but they can’t provide the same detail.

There could, however, be Antarctic ice still around from that particular interval of antiquity. The hard part is finding it, which requires recognizing it. Now, some researchers in Oregon State have used radioactive krypton to figure out the age of ice, a technique that may help us spot older samples.

The age of ice

Most ice cores are drilled at high points in the ice sheets, called “domes.” The ice is piled thick here, and just as importantly, it isn’t contorted by flow. When it’s thick enough, ice spreads under its own weight, a bit like pancake batter, but there’s actually no lateral flow right at the center.

But the lack of distortion doesn't mean that the record is easy to read deep into the core. The closer to the bottom of the ice sheet you get, the more the ice is compressed. While layers can simply be counted down to a considerable depth, they soon become less visible and identification gets more complicated.

This job would be easier if there were another way of measuring age. If you’re lucky enough to find a layer of volcanic ash in the ice, it can be dated, but there are only so many eruptions that spread ash to the poles. There have been attempts to apply radiometric dating using other things that can be found in the ice—like carbon-14, uranium-238, and argon-40—but all suffer from large error bars or confounding complications.

The noble gas krypton presents another possibility. One unstable isotope, krypton-81, is produced by collisions between cosmic rays and other isotopes of krypton in the atmosphere. Its half-life is 229,000 years, meaning it can be used to date air samples as old as 1.5 million years. The problem is that it’s only present in very small amounts, meaning you need a lot of air to make an accurate measurement. The amount of ice in a core won’t cut it.

Cores aren’t the only way to get old ice, though. There are also areas, especially at the end of the glacier, where flow brings old ice to the surface. In these places, you can essentially walk to the “Two-Mile Time Machine," climbing older layers up to younger ones. But while you can tell their relative ages, it's hard to tell how old they are relative to the present. Without being able to clearly count back from the present day like you can in an ice core, figuring out the age of that ice is trickier—especially as the layers have been disturbed by flow.

In order to do that, scientists have had to compare the measurements used as climate records to the well-dated ice cores. It’s a bit like taking an unlabeled week of Google’s stock prices and comparing it to the full chart to figure out which week you have.

A team led by Oregon State University researcher Christo Buizert tested the usefulness of krypton dating in that situation. They collected ice from the end of Taylor Glacier in Antarctica’s McMurdo Dry Valleys, just a short helicopter ride from McMurdo Station. Samples—about 350 kilograms apiece—were collected a few meters below the surface at four locations on the ice.

The samples were first dated using the standard method—comparison to nearby ice cores. The researchers then very carefully measured the isotopes of krypton trapped in the ice. The oldest sample was dated in the standard way at 123,500 ±4,200 years old. The error bars on the krypton dates are pretty large, but the technique yielded an accurate age of 120,000 ±26,000 years. The other three samples had matching ages within the error as well.

Those results are good enough to be of use in finding Antarctic ice older than existing cores. They could also come in handy for other investigations that take advantage of exposed ice, like isotope analyses of low-concentration gases that require large samples.

Age isn’t just a number

Aside from the interests of scientific research, these techniques also demonstrate how much robust evidence we have against notions that the Earth is only a few thousand years old. The annual layers of ice cores, better than tree rings, can show how completely incompatible this idea is with reality. Nevertheless, proponents of a young Earth frequently attack the reliability of the radiometric dating techniques that allow us to put exact numbers on geologic history.

Multiple radiometric dating methods provide numbers consistent with the simple layer counting of glacial ice—and even the calculations of the cycles in Earth’s orbit that drove the ice ages. It's not just that one of the methods is reliable; it's that all the evidence hangs together. The lower layers of ice at Taylor Glacier are clearly more than a few thousand years old—and now we have yet another method that can show us that fact.